Alloys and Intermetallic Compounds of Europium

Apart from a few general rules, the alloying behaviour of metals is rather empirical. The classical rules of Hume-Rothery [220] explain this behaviour reasonably well. Such factors as size, electronegativity, valency, electron concentration, free energy, formation of intermediate phases and isomorphism are found to influence the alloying tendency of metals. However, size and electronegativity are the two most important factors, and they profoundly influence the solubility of the solute atoms and greatly affect the crystal structures of the alloys. 

The classical theory of Hume-Rothery states that a difference in atomic diameters of solute and solvent atoms of more than 15% produces restricted solid solubility. The closest distance of approach of the atoms in the crystals of the element is taken as a measure of the atomic size. Substitution of a larger atom into a lattice requires a high amount of energy due to the concomitant disorganization of the parent lattice. However, the size factor becomes less important [221] when the difference in size is 8% or less. It is desirable (though not essential) that the size factor and the crystal structure of the elements producing a solid solution in all proportions be favourable. It is, however, apparent that if elements forming alloys did not possess the same crystal structure, a continuous series of solid solutions would be impossible. 

A generalization of the concepts of size factor and electronegativity has been made by Darken and Gurry [222] who plotted electronegativity 

It will be seen from the figure that the rare earths fall near the edge of the larger ellipse suggesting that they may exhibit only limited solubility in europium. It is interesting that Mg and Th, which dissolve the other rare earths to an appreciable extent, are apparently excluded. 

It is rather unfortunate that only a very limited amount of information on europium alloys and intermetallic compounds is available in the literature. In the following pages an attempt has been made to collect the available data on alloys containing europium. 

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No experimental data for the europium-scandium phase diagram were found. However, Miedema (1976), using Gsehneidneris (1969) value for the energy difference between divalent and trivalent europium (96kJ/g-at) found his method would account for the known information on the valence state of europium in intermetallic compounds. The application of his thermodynamic calculations to the europium-scandium alloy system predicted that no stable compounds exist in this system. 

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